It is well known that fluorine-doped tin oxide layers on glass surfaces decrease the electrical resistance of the thusly coated surfaces and increase the infrared reflection. To produce these tin oxide surface layers, a suitable tin compound and a fluorine-emitting compound are brought simultaneously into contact with the surface which is heated to temperatures of 400.degree. to 800.degree. C. The tin compound (base compound) forms then a coherent layer of tin oxide on the surface of the glass or the glass-ceramic. The fluorine-emitting compound (doping agent) causes the tin oxide layer which is being or has been formed to be doped with fluorine. This doping is of decisive importance for the development of the desired properties, such as conductivity and infrared reflection.
The method for producing these fluorine-doped tin oxide layers on suitable surfaces can be carried out in various ways. The tin- and fluorine-containing compounds may be allowed to act from the gas phase on the surface to be improved or the surface may be subjected to a stream of gas, which contains the compounds in the form of a fine powder. From a process technology point of view, the spraying of solutions of these compounds in a suitable solvent is particularly easy to carry out.
This art is dealt with extensively in the patent literature, from which the following Offenlegungsschriften or patents are named as representative.
German Patent 3,010,077 discloses a method for the application, by pyrolysis, on a glass surface of an infrared-reflecting layer of tin oxide, doped with a halogen, preferably fluorine. In this method, an organic tin compound (base compound) in powder form with a particle size of less than 20 .mu.m is applied as a suspension in a gaseous carrier stream in the presence of a halogen compound (doping agent) on the glass surface having a temperature of 400.degree. to 650.degree. C. As halogen compound (doping agent), a pulverulent compound is used, which is suspended in the gaseous carrier for the tin compound (base compound). An example of a base compound is dibutyl tin oxide, while dibutyl tin difluoride is named as doping agent. Base compound and doping agent may also be realized in a single compound, such as ammonium dibutyl tin tetrafluoride. However, it has been ascertained that it is extremely difficult to achieve a reproducible formation of a uniform flow of carrier gas and pulverulent compounds, so that surface layers with constantly changing properties are obtained.
German patent 2,806,468 teaches a method for which dibutyl tin oxide, in the form of a powder dispersed in an anhydrous stream of air, is used as base compound, while the doping agent is a gaseous fluorine compound, which is mixed in with the dispersion shortly before it reaches the discharge nozzle. Anhydrous hydrogen fluoride, BF.sub.2, BrF.sub.5, ClF.sub.3, SiF.sub.4, WF.sub.6 and MoF.sub.6 are named as gaseous fluorine compounds. These gaseous fluorine compounds have the tendency of etching the surface of the glass or glass-ceramic that is to be improved. Moreover, extensive safety precautions have to be taken when using these chemically aggressive and toxic gases. This renders the procedure cumbersome and expensive.
The application of suitable tin compounds and doping agents from the gas phase can be deduced from the European Offenlegungsschrift 0 112 780. In this method, a gaseous mixture of butyl tin trichloride (base compound) and dichlorodifluoromethane (doping agent) is used. Here also, metering problems arise, which lead to surfaces of uneven and inadequate properties. Moreover, the use of dichlorodifluoromethane is undesirable for environmental reasons. A method is disclosed in U.S. Patent 4,293,594, wherein a gaseous mixture of dimethyl tin dichloride and dimethyl tin difluoride is applied in a carrier gas on the surfaces to be improved. However, these compounds are solids and can be converted into the gas phase at high temperatures only.
A liquid preparation for the production of high-grade, fluorine-doped tin oxide layers on glass surfaces is disclosed in the European Offenlegungsschrift 0 158 399. The preparation comprises
(a) 1 to 30% by weight of a doping agent based on an organic fluorine compound, selected from trifluoroacetic acid or its anhydride, ethyl trifluoroacetate, trifluoroethanol and pentafluoropropionic acid and
(b) 70 to 99% by weight of an organotin compound, selected from alkyl tin trichloride, dialkyl tin dichloride, alkyldichloro tin acetate, alkylchloro tin diacetate, an ester of tin chloride or tin tetrachloride. PA1 30 to 89% by weight of tin-I1; chlorides, which optionally comprise organic groups, PA1 1 to 5% by weight of tin-II fluoride, PA1 10 to 55% by weight of a polar organic solvent and PA1 0 to 10% by weight of water. PA1 30 to 89% by weight of t-n tetrachloride or alkyl tin trichloride, PA1 1 to 5% by weight of tin-II fluoride, PA1 10 to 55% by weight of a polar organic solvent and PA1 0 to 10% by weight of water. PA1 30 to 89% by weight of tin tetrachloride or butyl tin trichloride, PA1 1 to 5% by weight of tin-II fluoride, PA1 10 to 55% by weight of ethanol and/or ethyl acetate and PA1 0 to 10% by weight of water. PA1 30 to 89% by weight of tin tetrachloride or butyl tin trichloride, PA1 1 to 5% by weight of tin-II fluoride and PA1 10 to 65% by weight of ethanol.
This method also does not yet satisfy, since the majority of the fluorine-containing doping agents represent low boilding liquids and therefore largely evaporate at the hot glass surface and, for this reason, are not incorporated in the tin oxide layer. Due to the heat of evaporation of these doping agents, the hot substrate surface is cooled relatively strongly. This leads to a worsening of the properties of the applied layer.
If pentafluoropropionic acid is used, the decomposition at the hot glass surface does not occur quickly enough, so that the tin oxide layer also in this case does not have sufficient functional values. In addition, these compounds are very expensive and injurious to health.